Short Version.
I think there should be at least one level progression for workers. They are doing labour, and even the most unskilled seeming labour is something that you can get skilled at - and the sheer movement alone of mining, building and collecting (even harvesting wild plants) should be enough for them to be able to make even the most tiny progression. This would also allow you to leave characters as workers for longer periods knowing that you weren’t hurting their overall stats (As in, health, speed …etc) progression until they reach the max in which point they would need to move to something more skilled to progress further.
Ideally there would be 3 levels of it - but one level would be enough in my opinion.